Digital communication with mobile receivers is becoming increasingly significant with the proliferation of cellular telephones and mobile data terminals in our society. This trend is likely to continue. Some advantages of digital communication over analog communication are: Improved utilization of the radio spectrum; Increased reliability in communicating information which is very sensitive to channel errors; Direct access to computerized databases; The relative ease of protecting confidentiality and integrity by the use of encryption techniques; and Integration of voice and data services.
In most urban and many suburban areas, a major obstacle to achieving efficient and reliable data communication over a VHF or UHF mobile radio channel is multipath propagation. Multipath propagation results in severe and rapid fluctuations in the received signal strength as the mobile receiver is moved. The duration of a fade depends on the velocity of the receiver and is typically on the order of a few milliseconds.
The rate of fluctuation in the strength of the received signal is characterized by the Doppler rate f.sub.d which is given by f.sub.d =f.sub.c v/c, where f.sub.c is the carrier frequency, v is the vehicle speed, and c is the velocity of light.
The variation in amplitude of a received fading signal is well approximated by a Rayleigh distribution over short distances of a few tens of meters. Consequently, fading caused by multipath propagation of a signal is called Rayleigh fading. Over larger distances, shadowing of the received signal by hills and other large obstacles results in a log-normal variation in the mean of the Rayleigh distribution. Rayleigh fading imposes severe constraints on digital communication.
In a conventional serial modulation scheme data bits are transmitted over a channel sequentially. If a deep fade occurs during the transmission of such a signal then the bits which are transmitted during the deep fade may not be received. This problem can be reduced by transmitting a data frame containing a block of bits in parallel, each at a low baud rate so that the time taken to transmit the frame is long (typically, for example, on the order of a fraction of a second) relative to the expected duration of a fade. The effect of a fade is then spread out over many bits. Rather than a few adjacent bits being completely destroyed by a fade, all of the bits in the frame are slightly affected by a fade which occurs during the time that the frame is being transmitted.
A good scheme for transmitting a block of bits in parallel over a channel is orthogonal frequency division multiplexing (OFDM). OFDM may be used in many different types of data channel. The data channel may comprise, for example, acoustic signals being propagated through water, analog signals being transmitted through a wire such as a telephone line, AM, FM, or Single Side Band (SSB) radio signals. OFDM/FM is particularly attractive because it can be implemented by retrofitting existing FM communication systems.
In OFDM a block of bits is transmitted in parallel through a channel comprising a number of sub-carrier frequencies. The sub-carrier frequencies are chosen to be spaced in frequency from each other by a multiple of the symbol rate. That is, if the OFDM symbol duration is T seconds, the sub-carrier frequency spacing is 1/T Hz. With this frequency spacing, the sub-carriers are orthogonal over one symbol interval.
Data to be transmitted are grouped into blocks of K bits. Each block of K bits is encoded, as is further described below, and transmitted as a single data frame. The block of K bits is generally divided into smaller groups, each smaller group usually containing between 2 and 5 bits. Each smaller group is assigned to one sub-carrier frequency. The phase and magnitude of the sub-carrier at that frequency are then set to values determined by the data represented by the small group of bits. The encoding scheme is chosen to provide at least 2.sup.m discrete signal points differentiated from each other in phase and/or magnitude where m is the number of bits assigned to the sub-carrier. For example, each of the 2.sup.m possible combinations of the m bits assigned to a sub-carrier frequency may be assigned to one of the 2.sup.m signal points in a 2.sup.m -QAM (Quadrature Amplitude Modulation) constellation. The number of bits m assigned to each sub-carrier can vary from one sub-carrier to the next. Of course, other encoding schemes besides QAM are also possible.
The transmitted signal is received by a receiver. For each sub-carrier, the transmitted information is extracted by measuring the phase and amplitude of each data-carrying sub-carrier and determining which point in the 2.sup.m -QAM constellation is closest to the signal point corresponding to the received sub-carrier. This signal point then identifies the m-bit data sequence transmitted on that sub-carrier. The original K bit block of data can then be reconstructed by combining the bits of data recovered from each sub-carrier.
Because information is encoded in the phase of the transmitted signal it is necessary to provide a means for synchronizing the received signal with the transmitted signal. Furthermore, particularly in a pure ALOHA environment, it is necessary to synchronize each OFDM frame independently. In a pure ALOHA environment the receiver does not know when a data frame will be transmitted. A data frame may be transmitted at any random time. Therefore, in a pure ALOHA environment the receiver cannot rely on information provided in previously received or subsequently received data frames for obtaining synchronization information for recovering data from a received data frame. Each data frame must carry its own synchronization information. Because of this limitation, it is difficult using prior art techniques known to the inventors to provide efficient OFDM communications channel in a pure ALOHA environment. As the available spectrum is limited, any synchronization scheme should have a low bandwidth overhead.
Synchronization schemes devised for parallel (OFDM) transmission over telephone channels have been disclosed in Hirosaki, A 19.2 kbps voiceband data modem based on orthogonally multiplexed QAM techniques, IEEE International Conference on Communications, 1985, Chicago Ill.; Keasler, Reliable data communication over the voice bandwidth telephone channel using orthogonal frequency division multiplexing, Ph. D. Thesis, University of Illinois at Urbana-Champaign, 1982 and Baran, U.S. Pat. No. 4,438,511. However, these schemes rely on several consecutive frames to maintain correct timing and do not meet the requirement that synchronization is achieved for each frame individually.
Moose, Differential modulation and demodulation of multi-frequency digital communications signals MILCOM 90, pp. 273-277, October 1990 discloses a synchronization technique for use in mobile satellite communications. In this technique, a synchronization frame is used to provide synchronization for a group of following data frames.
The performance of a synchronization technique can be measured by observing the frequency of false alarms, mis-detections and bad synchronizations. A false alarm occurs if the synchronization algorithm indicates the presence of a data block when none is present. A mis-detection occurs if the synchronization algorithm does not detect the presence of a data block when one is present. A bad synchronization occurs if the synchronization algorithm detects the presence of a data block when one is present, but does not correctly synchronize. Bad synchronization occurs when the signal is distorted enough to prevent proper synchronization, but not so distorted as to cause a mis-detection. A correct synchronization occurs if the synchronization algorithm detects the presence of a data block when one is present and correctly synchronizes to the data block.
Preferably a synchronization system should perform sufficiently well that inaccuracy in the synchronization procedure is not the major factor limiting the achievable bit-error-rate (BER). That is, the BER of the communication channel should be limited by the OFDM modulation technique rather than by the synchronization procedure. For example, in designing the synchronization scheme, the target probability of false alarm, mis-detection or bad synchronization may be set equal to the BER of the OFDM/FM system given ideal synchronization. This ensures that the BER with the synchronization procedure does not exceed twice the BER given ideal synchronization.